Acid site generation

In addition, if we accept the conventional mechanism of Brpnsted acid site generation by hydrogen spillover from Pt sites, as proposed for Pt-S04=/Zr02, we must recognize that this would be in fact a reduction process (zerovalent hydrogen yielding -OH groups). 

Sato et al. [195] have studied the surface borate structures and the acidic properties of alumina-boria (3-20 wt.%) catalysts prepared by impregnation method using B(MAS)-NMR measurements and TPD of pyridine, as well as their catalytic properties for 1-butene isomerization. The number of Brpnsted acid sites was found to increase with increasing boria content, and the catalytic activity was explained by the strong Brpnsted acid sites generated by BO4 species on the surface of alumina. 

From the general inaccessibility of both the sodium and TMA ions, we postulate that most of the acidic sites generated by thermal treatment of the derived NH4+/TMA+ zeolite will also be inaccessible to reactant molecules. Likewise, catalytically active metals such as Pt and Pd introduced by ion exchange are expected to be located in or near these same inaccessible sites. This may explain the poor approach to equilibrium observed with the isomerization catalysts, and the poor hydrogenation activity of the hydrocracking catalyst indicated by excessive coking and catalyst decline, even in the presence of a massive 3.1 wt % palladium. 

The strength of the acid sites generated on the AMM samples is characterized by NH3-TPD. As shown in Figure 4, the amount of NH3 adsorbed on PSM is very low. After alumination, the amount of adsorbed NH3 increases with the amount of A1 species on the AMM samples. This increased amount of NH3 adsorbed on AMM samples shows that more acid sites have been created upon post-synthesis alumination. This result is in good agreement with that of FT study, where both the amount of Lewis... 

The data reported in Figure 6.5 indicate that the adsorption process of NH3 in the natural mordenite, MP, is a heterogeneous process. In the plot, a first value at very high energy is evident, which should be related to the acid-base interaction of the NH3 molecule with acidic sites generated in the natural mordenite during the activation [31,49],... 

The protons thus formed as above are present as surface hydroxyl groups and is a cause of Broensted acidity in zeolites. This accounts for the high acidity and catalytic activity of HY. CuY showed a better activity than NaY. This as probably due to Cu" ions which constitute Lewis acid sites generating Broensted acid sites during activation treatment of CuY carried out prior to the dehydration reaction. The precursor for the above protons and hence the new Broensted sites could be the molecular water that gets evolved during the activation treatment as seen from TG/DTA profile. 

The Bronsted acid site FT-IR band intensity (at 1491 cm i) directly correlates with the catalytic achvity, and the maximum activity observed for 19 wt% WZ is due to the maximum number of Bronsted acid sites generated by the higher interaction of tungsten(lV) oxide with zirconia. Similar composite materials were studied in the acylation of 2-MN with AAN to selectively give the 1-acetylated product in 50% yield. ... 

In the SAPO-5 samples, the number of acid sites generated per Si appears to change dramatically with the Si concentration. For the dilute case, SAPO-5(0.5), there appears to be approximately one site/Si. This sample was also the most sensitive to calcination conditions, with the concentration of sites measured following calcination in laboratory air being only a tenth of that reported above. Both of these facts suggest that the Si are entering the framework as isolated sites, with most of the Si atoms replacing framework P. For the more concentrated case. 

SAP0-5(17), the number of acid sites generated is only a small fraction of the number of Si atoms. This sample was also much less sensitive to calcination conditions, exhibiting approximately half the sites when calcined in laboratory air. Most of the Si appear to be entering the framework as pairs, displacing an Al-P pair. These Si islands generate fewer acid sites and are likely less sensitive to the presence of water during calcination. The tendency to form Si islands in SAPO s has been reported previously [10]. 

Summarizing the reports of acid strength of AlPO-n-based molecular sieves, it can be concluded that the number of acid sites generated has a complex relationship to the number of metal cations substituted into framework, and the acid strength is generally... 

With the exception of iron, this procedure is simple and cheap, but the complete prevention of TM hydrolysis and Bronsted acid site generation is not allways possible. Residual acidity will catalyse peroxide decomposition and is therefore detrimental to the use of the material as oxygenation catalyst. The preferential occupation of hidden sites of (bivalent) TM ions in presence of an excess of monovalent alkali cations, will often cause incomplete reaction with DCB. Residual TM in oxygenation reaction conditions will cause radical decomposition of the peroxide reagent and be at the origin of radical chain reactions of the substrate with dioxygen thus formed. Rnally, only microporous materials with cation exchange capacity can be used as support. 

The liquid phase Beckmann rearrangement of cyclohexanone oxime 5 in the presence of a series of mesoporous molecular sieves with various Si02/Al203 ratios has been reported. The surface silanol groups ineffectively catalyzed the rearrangement, whereas the acid sites generated by incorporation of aluminium improved the activity and selectivity to lactam 6. 

For a certain combinations of metal oxide, the number of acid and base sites generated usually depends upon the composition of the binary oxide. Examples are shown in Fig. 3.46 for acid site generation of Si02 MoOs, and Fig. 3.47 for base site... 

The above pattern of acid site generation and elimination is observed not only for... 

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